Drosophila has become an important model system for the study of innate immunity, in particular the humoral (cell-free) response to bacterial and fungal infections. Drosophila is also attacked by parasitoid wasps, which lay their eggs in fly larvae. Fly larvae mount an immune response in which hemolymph (blood) cells attach to and encapsulate the wasp egg, deposit a layer of melanin around the egg, and release cytotoxic molecules within the melanized capsule to kill the egg. The molecular basis for this response, and for the cellular immune response in general, is poorly understood. Nor is it understood how the innate immune system distinguishes host tissue from that of closely related parasites. I will use genetic mapping, mRNA gene expression, and other methods to determine the genetic basis for resistance, and variation in resistance, against a parasitoid wasp in D. melanogaster. Furthermore, I will measure the effect of variation in DNA sequence and transcription levels of known genes of the evolutionarily conserved phenoloxidase pathway, responsible for the generation of melanin, on the efficiency of melanin deposition in Drosophila. [unreadable] [unreadable]